Gas-filled bubble seismo-acoustic source

ABSTRACT

A sound source includes a tubular resonator configured to be filled with a gas. The exterior of the resonator includes rigid and elastomeric portions. The interior of the resonator includes a first volume and a second volume. The volumes are separated by a rigid tubular wall containing at least one orifice. The at least one orifice enables a flow of gas between the volumes. The resonator also includes at least one rigid tubular member configured to move along the rigid tubular wall. The position of the at least one rigid tubular member regulates at least one dimension of the path between the volumes. The sound source also includes a volume velocity actuator disposed within the resonator. The sound source also includes a processing circuit configured to provide a control signal to cause the volume velocity actuator to perturb the gas within the resonator at a defined frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/053,095, filed Mar. 21, 2011, and titled “Gas-Filled Bubble SoundSource.” This application also claims the benefit of U.S. ProvisionalApplication No. 61/597,150, filed Feb. 9, 2012, and titled “SoundSource.” The entireties of U.S. application Ser. Nos. 13/053,095 and61/597,150 are hereby incorporated by reference.

BACKGROUND

Low frequency acoustic and seismo-acoustic projectors find applicationsin underwater ocean acoustic tomography, long-range acoustic navigationand communications and deep-bottom penetration seismic profiling in theoffshore oil and gas industry. Such sources may be used in Arcticunder-ice acoustic far-range navigation and communications, underwaterglobal positioning systems (RAFOS), and long-range ocean acoustictomography and thermometry. Low-frequency underwater sound sourcesshould be powerful and efficient.

The low frequency source can be an explosive (dynamite), or it can usemore complicated technology like an air gun providing single pulses, orlike vibroseis providing continuous frequency sweeps. Some acousticsources in use for seismic applications, such as air gun, plasma(sparker) sound sources and boomers, are of the impulse type, where thetransmitter emits a large non-coherent pressure pulse during a shorttime interval. Seismic air-gun surveys, such as those used in theexploration of oil and gas deposits underneath the ocean floor, produceloud, sharp impulses that propagate over large areas and increase noiselevels substantially. Their signal is not highly controllable, either infrequencies content or repeatability. Coherent sound sources such asmarine vibroseis can be much quieter and potentially less harmful formarine environments and should be used instead of air-guns in certainexploration activities. Current continuous wave type sources make use ofhydraulic, pneumatic, piezo-electric or magneto-strictive drivers anddifferent type of resonance systems to store acoustic energy and toimprove impedance matching, when generating low-frequency sound waves inwater. The power output of a simple acoustic source is proportional tothe squares of volume velocity and frequency and needs a large vibratingarea to achieve reasonable levels. As a result, the sound source canbecome unacceptably large and expensive.

SUMMARY

According to one embodiment, a sound source comprises a bubbleconfigured to be filled with a gas. The sound source further comprises avolume velocity actuator configured to perturb the gas within the bubbleby changing the volume of gas within the bubble. The sound sourcefurther comprises a processing circuit configured to provide a controlsignal to the volume velocity actuator to cause the volume velocityactuator to perturb the gas within the bubble at a frequency defined bythe control signal. The sound source further comprises a resonantfrequency control mechanism configured to keep a resonant frequency anda phase of a radiated signal of the bubble approximately equal to afrequency and a phase of the control signal.

According to another embodiment, a sound source comprises a tubularresonator configured to be filled with a gas. The exterior of theresonator comprises rigid and elastomeric portions. The interior of theresonator comprises a rigid tubular wall containing at least oneorifice. The interior of the resonator further comprises a first volumeand a second volume. The first volume and the second volume areseparated by the rigid tubular wall containing at least one orifice. Theat least one orifice enables a flow of the gas between the first volumeand the second volume. The interior of the resonator further comprisesat least one rigid tubular member configured to move along the rigidtubular wall. The position of the at least one rigid tubular memberregulates at least one dimension of the path between the first volumeand the second volume. The sound source further comprises a volumevelocity actuator disposed within the resonator and configured toperturb the gas within the resonator. The sound source further comprisesa processing circuit configured to provide a control signal to thevolume velocity actuator to cause the volume velocity actuator toperturb the gas within the resonator at a frequency defined by thecontrol signal.

According to another embodiment, a method of generating underwater soundwaves comprises providing a tubular resonator configured to be filledwith a gas into an underwater environment. The end portions of thetubular resonator are covered by an elastic membrane. The tubularresonator comprises at least two sections separated by a rigid tubularwall. The rigid tubular wall contains a plurality of openings connectingthe two sections. The tubular resonator further comprises at least tworigid tubular members symmetrically disposed along the rigid tubularwall. The at least two rigid tubular members are configured to moverelative to the plurality of openings. The tubular resonator furthercomprises a volume velocity actuator disposed within the resonator andconfigured to perturb the gas within the resonator. The tubularresonator further comprises a processing circuit configured to provide acontrol signal to the volume velocity actuator to cause the volumevelocity actuator to perturb the gas within resonator at a frequencydefined by the control signal. The method further comprises perturbingthe gas within the resonator. The method further comprises controllingthe perturbing of the gas within the bubble to emit sound waves over aplurality of frequencies. The method further comprises controlling aresonant frequency and a phase of the resonator to approximately equal afrequency and a phase of the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a sound source and control system,according to an exemplary embodiment.

FIG. 2 is a graph depicting the sound pressure level (SPL) of a bubblesource with radius varying from 0.1 m to 1 m at a depth of 100 meters,according to an exemplary embodiment.

FIG. 3 is a graph depicting the sound pressure level (SPL) of a 50 Hzbubble source with radius 0.8 m and actuator volume displacement 67 ccat the depth 1500 meters, according to an exemplary embodiment.

FIG. 4 is a graph depicting a difference of phase between a radiatedsignal and a control signal of the bubble resonator with radius 0.8 m atthe depth 1500 meters, according to an exemplary embodiment.

FIG. 5 is a picture of a lift bag suitable for use with the sound sourceof FIG. 1, according to an exemplary embodiment.

FIG. 6 is an illustration comparing exemplary sizes of low frequencysound sources to a size of an exemplary bubble sound source.

FIG. 7 is a flowchart of a method of generating underwater sound waves,according to an exemplary embodiment.

FIG. 8A is a diagram of an actuator, according to an exemplaryembodiment.

FIG. 8B is a diagram of an actuator, according to another exemplaryembodiment.

FIG. 9 is a block diagram showing a sound source and control system,according to an exemplary embodiment.

FIG. 10A is graph depicting the sound pressure of the sound source ofFIG. 9.

FIG. 10B is a graph depicting the difference of phase between a radiatedsignal and a control signal of the sound source of FIG. 9, according toan exemplary embodiment.

FIG. 11 is a diagram of an electrical circuit that may be used tosimulate the sound source of FIG. 9, according to an exemplaryembodiment.

FIG. 12 is a graph depicting simulated sound pressure levels of thesound source of FIG. 9, with sleeve displacements varying from 0.01 m to0.5 m, using the model electrical circuit of FIG. 11, according to anexemplary embodiment.

FIG. 13A is a block diagram showing a sound source and control system,according to an exemplary embodiment.

FIGS. 13B-F are alternate views of the sound source of FIG. 13A,according to exemplary embodiments.

FIG. 13B is a cross-sectional view of the sound source of FIG. 13A,according to an exemplary embodiment.

FIG. 13C is a cross-sectional view of the sound source of FIG. 13A froman elevated angle, according to an exemplary embodiment.

FIG. 13D is a view of the exterior of the sound source of FIG. 13A froman elevated angle, according to an exemplary embodiment.

FIG. 13E is a cross-sectional view of the sound source of FIG. 13A fromdirectly above the sound source, according to an exemplary embodiment.

FIG. 13F is a cross-sectional view of the sound source of FIG. 13A fromthe side and along the length of the sound source, according to anexemplary embodiment.

FIG. 14 is a diagram of an electrical circuit that may be used tosimulate the sound source of FIG. 13A, according to an exemplaryembodiment.

FIG. 15 is a graph depicting the simulated sound pressure levels of thesound source of FIG. 13A, with sleeve displacements varying from 0.01 mto 0.5 m, using the model electrical circuit of FIG. 15, according to anexemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some embodiments described herein may provide a simple and lessexpensive engineering solution for a large volume, low-frequencyresonance system. Some embodiments described herein may provide animproved radiated power and/or an improved radiated impedance, asreflected to a driver. Some embodiments described herein may provide forincreased electro-acoustical efficiency. Some embodiments describedherein may provide for a lighter, smaller, and lower cost sound source.Some embodiments described herein may provide a coherent signal that isless damaging to marine life.

Some embodiments described herein may enlarge the frequency band coveredby a resonator by tuning a narrow-band resonator over a large range offrequencies. Some embodiments described herein may be a coherent type ofsound source suitable for 5-100 Hz frequency range. Some embodimentsdescribed herein may cover the 5-100 Hz frequency band by tuning anarrow-band, high efficiency sound projector over the necessaryfrequency band. Some embodiments described herein may be tuned withoutchanging buoyancy, which may be more convenient when the resonator istowed. Some embodiments described herein may work at lower frequencieswith the same physical dimensions. Some embodiments described herein mayhave no moving parts in water, which may increase the reliability of theresonator.

Referring first to FIG. 1, a gas-filled bubble sound source or resonator20 is shown. Source 20 comprises a bubble, balloon or air bag 1, whichmay be a gas-filled bubble sound source. The gas may be air, Nitrogen,or other gases. For example, Nitrogen may be used for deep water and airmay be used for shallow water. Bubble 1 may be manufactured from anelastic material, such as fiber reinforced latex, chloroprene, neoprene,buna rubber, etc. The elastic material may be selected to have a verylow level of gas leakage for long-term deployments. Source 20 comprisesa base, end-cap or interface 6 configured to seal a portion of bubble 1and to further provide conduits for electrical and mechanical inputs andoutputs to bubble 1. Source 20 may be a coherent source of changeable orchanging volume velocity (velocity per area).

Source 20 comprises an actuator or driver 5 shown in this embodiment inthe form of a symmetrical piston system placed or disposed inside orwithin the bubble volume (though it may be disposed outside of or on asurface of the bubble in alternative embodiments). Actuator 5 may be anyactuator configured to perturb bubble 1, for example in a manner thatwill cause bubble 1 to vibrate or oscillate so that energy will beexchanged between the pressure of the gas inside and the inertia of thewater at the surface of bubble 1. Actuator 5 may be amechanically-driven actuator in this exemplary embodiment. Actuator 5comprises a crankshaft 3 driven by an electrical motor 2 which in turndrives a plurality of symmetrically moving pistons 4. Electrical motor 2may be a rotary motor with a crankshaft or a linear motor attacheddirectly to moving pistons. Two moving pistons are provided in thisembodiment, though three, four, or more pistons or other moving membersmay be used in alternative embodiments. In this embodiment, pistons 4are configured to move synchronically in opposite directions. Themoveable members may be configured to move in a same plane but opposedin any direction. Actuator 5 may be configured to make very smallchanges in pressure within bubble 1, relative to pressure changes madeby the gas supply system described below. A housing 22 may be acylindrical housing or other shape configured to define an internal areabetween pistons 4 sealed for air pressure. The internal area issubjected to a fluctuating pressure from the moving pistons 4. Theelectrical motor driver 2 is controlled by control signal from acomputer (processing circuit 14) and synchronized with a digitallysynthesized signal waveform. The waveform of the control signal may be asine wave, or other waveform. The processing circuit 14 may beconfigured to generate the control signal in response to a programmedalgorithm set by an operator of source 20, for example to control source20 to provide a single frequency output, plurality of frequency outputs,etc. over one or more time periods. In one example, the actuator 5 isdisposed within the bubble, for example, in a configuration where watersurrounding bubble 1 does not come into contact with the actuator, or incontact with either side of housing 22 or with either of pistons 4.

Source 20 further comprises a computer controlled gas supply system 24configured to regulate the volume and/or pressure of gas inside bubble 1in proportion to a transmitted signal from a processing circuit 14. Gassupply system 24 comprises a tank 9 with compressed air or liquidNitrogen connected to bubble 1 with air pipes 7 via a solenoidcontrolled valve 8. Tank 9 may be disposed proximate bubble 1 or via anextended conduit or hose to a pump on a ship deck on a surface of thewater. Gas supply system 24 is configured to fill bubble 1 with gas fromexternal tank 9 or from the ship through an underwater air hose and torelease gas through a solenoid controlled release valve or vent 11 andpressure release pipe 10. Gas supply 24 may further comprise aprocessing circuit 14 configured to control solenoids S of valve 8 andvent 11 and an electrical driver 13 configured to drive actuator 5.Processing circuit 14 may be coupled to a precision clock 15. Theprecision of clock 15 may depend on stability demand. For example, atemperature-compensated crystal oscillator (TCXO) provides ±1 ppmfrequency stability over the −40° C. to +85° C. industrial temperaturerange. As another example, clock 15 may be a Rubidium or Cesium atomicclock with stability better than 0.0001 ppm. As another example, a chipscale atomic clock (CSAC) may be used, such as a Symmetricom SA.45shaving a precision of ±5.0E-11.

A sensor, microphone, or hydrophone 12 is coupled to an analog-digitalconverter (ADC) input of processing circuit 14. Sensor 12 is disposedinside bubble 1 and provides a feedback signal to processing circuit 14.Processing circuit 14 is configured to keep a resonance frequency ofbubble 1 approximately or substantially equal to a central frequency ofa signal transmitted by bubble 1, by controlling gas volume inside thebubble using solenoids and valves 8, 11. A phase difference between asignal from sensor 12 and a signal sent to control actuator 5 is used asan indicator of difference between the resonant frequency of bubble 1and central frequency of emitted signal. Sensor 12 is coupled through anamplifier 16 to a phase comparator 17 having as its inputs the amplifiedsignal from sensor 12 and the signal sent to control actuator 5 (or asshown to control electrical driver 13 to control actuator 5), and havingas its out put a compared signal sent to processing circuit 14 forfurther processing as part of a phase feedback loop. Gas supply system24 can be used to keep bubble 1 in resonance with an instantaneousfrequency of a linear frequency modulated signal sweeping in a largefrequency bandwidth for high precision bottom penetration profiling.

A spherical pressure gas-filled underwater bubble or balloonmanufactured from an elastic material offers a large radiating area. Theradius of the bubble and its surface area depend upon depth andfrequency: for example, at a 1500 m depth, a 2 m radius bubble will havea resonant frequency of 20 Hz, and a bubble area of 50 square meters;for a frequency of 150 Hz, the radius will be 0.275 m and have an areaabout 0.95 square meter. For the sound source depth of 100 meters, abubble with radius 1.1 meter and area 15 square meters will have 10 Hzfrequency resonance, and a bubble with radius 0.11 meters and area only0.15 square meter will have a resonance of 100 Hz. In one embodiment,the bubble can be less than or equal to about 2 meters radius and lessthan or equal to about 50 square meters area and greater than or equalto about 0.11 meter radius and greater than or equal to about 0.15square meters area. In alternative embodiments, radii and surface areasmay be greater than or less than these sizes. The bubble may function asa good impedance transformer, which increases the resistive component ofthe radiation impedance. The radiated impedance of the bubble projector,as reflected to the actuator or driver, is larger than for a directradiator, which implies that the radiated power and theelectro-acoustical efficiency are increased. The driver for the bubbleprojector may supply greater blocked pressure and reduced volumedisplacement than the direct emitter when both are operating at the sameradiated power. To achieve high efficiency, a symmetrical air pump withopen cylinders may be used having a closed central part between thecylinders, driven by an electrical motor controlled by a computer, andsynchronized with digitally synthesized signal waveforms. When the airpump driver has just a closed central part (embodiment of FIG. 9A), theair pump has no resonance. This embodiment is suitable for use to sweepover a large band by changing exiting frequency and tuning the bubble bychanging its dimensions. When the air pump is configured as in theembodiment of FIG. 9B, it will have resonance because the central partwill have a form of Helmholtz resonator. The central part of the drivermay have a form of Helmholtz resonator with its own resonant frequency.This embodiment is suitable for use to expand bandwidth in a case wherethe sound source may not by sweeping but may be using broadband signalswith the central frequency in the middle of the frequency band of thesound source. The bubble resonant frequency and driver resonantfrequency may be closely grouped to form a doubly resonant projector.This provision may yield transmitting bandwidth of ½ octave or greater.

Referring now to FIGS. 2 and 3, solutions of equations for soundpressure of a bubble source are shown, according to two exemplaryembodiments. The pressure inside a bubble projector provided by ainternal source of volume velocity can be described by knowndifferential equations in the following form:

$\begin{matrix}{{\frac{^{2}p}{t^{2}} + {\frac{\omega_{r}}{Q}\frac{p}{t}} + \omega_{r}^{2}} = {\frac{P_{0}}{V_{0}}\frac{V_{a}}{t}}} & (1) \\{{\frac{\omega_{r}}{Q} = {\frac{\gamma \; P_{0}S_{0}}{V_{0}\rho \; c} = {\omega_{r}k_{r}a\mspace{20mu} {where}}}}\mspace{14mu} {\omega_{r}^{2} = \frac{\gamma \; P_{0}S_{0}}{V_{0}\rho \; c}}} & (2)\end{matrix}$

is the resonant frequency of a spherical bubble with a radius a, volumeV₀=(4/3)πa³, S₀=4πa² is the bubble surface area, and pressure P₀; γ(1.4) is the ratio of the specific heats at constant pressure tospecific heat at constant volume for gas within the bubble;k_(r)=ω_(r)/c is the resonance wave number; Q=1/(k_(r)a) is theQ-factor. The solution of equation (1) is straight-forward forsimulation and calculation of necessary volume velocity for an actuator.As shown in FIGS. 2 and 3, resonance frequent and inverted Q-factor areincreasing proportionally with the square root of pressure or depth. Thebandwidth of the bubble source can be potentially increased twice byadding additional resonance in the acoustical driver. Also, theresonance acoustical drive can be much more efficient.

FIG. 2 shows the sound pressure level (SPL) of a bubble source at depth100 meters, as the radius of the bubbles varies from 0.1 m to 1 m. FIG.2 shows how by changing bubble dimension, the resonant frequency can beswept. For example, in shallow water, the bubble resonator can becomevery high Q and low bandwidth, and to achieve a necessary bandwidth of10-100 Hz, the resonant frequency can be swept simultaneously or alongwith the signal frequency of the control signal to keep the bubble inresonance with the control signal. FIG. 3 shows SPL of a 50 Hz bubblesource with radius 0.8 m and actuator volume displacement 67 cc at thesame depth of 1500 meters.

The necessary level of volume velocity displacement (16.7 cc for thefirst example of FIGS. 2 and 66 cc for the second example of FIG. 3) atthe frequencies lower than 150 Hz can be achieved by a standardsymmetrical piston system, where pistons driven by an electrical motorare moving synchronically in opposite directions with an internal areabetween pistons sealed for air pressure. The electrical motor may be arotary motor moving pistons with a crankshaft mechanism or a linearmotor connected to pistons directly. The efficiency of such pumps may beas great as 30%. The brushless electrical motor (linear or rotary) witha computer controlled driver can reproduce a digitally synthesizedsignal.

The resonant frequency of a bubble resonator depends on internalpressure and on its volume and therefore may be controlled by pumpingair from an external pressure tank into the bubble or releasing air fromthe bubble to change the dimensions of the bubble. The processingcircuit 14 may be configured to change resonant frequency by pumping gasand expanding the bubble or by releasing gas and shrinking bubble,whereby one or more dimensions of the bubble are increased or decreased,respectively. To keep air-filled bubble 1 in resonance with a centralfrequency of a radiated signal and to keep the keep the phase of theradiated signal equal to the phase of the control signal, a microphonephased locked loop (PLL) can be applied (for example, as part ofprocessing circuit 14). A phase difference between internal bubblepressure and a transmitted control signal sent on line 26 is anindicator of resonance. Processing circuit 14 is configured to keepingthe phase difference close to zero in order to keep bubble resonatorsubstantially in resonance with the signal emitted by bubble 1. Inshallow water, when the Q-factor of the bubble resonator is very highand frequency bandwidth is too narrow for broadband signal transmission,the same PLL system can be configured to keep the bubble in resonancewith the instantaneous frequency of a slowly changing frequencymodulated signal. Such a system can be configured to sweep through apredetermined bandwidth, such as from 5 Hz to 100 Hz, and became acoherent replacement of widely used air-guns.

FIG. 4 shows how phase difference between a control signal (input signalto actuator 5) and internal bubble pressure indicates the resonantfrequency shift relative to the central signal frequency. The parametersof the bubble sound source are the same as in FIG. 3: resonant frequencyis equal to 50 Hz, bubble has radius 0.8 m, actuator volume displacementis 67 cc and depth is the 1500 meters. The internal sound pressure isproportional to a signal received from internal microphone 12, FIG. 1.The phase of that signal can be compared by a phase comparator (part ofprocessing circuit 14) with the phase of sound source input signal. Ifinternal pressure phase is larger than input signal phase (region A),then the resonant frequency of the bubble is higher than the frequencyof the emitted signal and processing circuit 14 is configured to controlthe system to pump air inside bubble and increase its radius to decreaseresonant frequency. If the phase of the microphone signal is lower thanthe phase of input signal (region B), then the resonant frequency of thebubble should be increased by releasing gas outside and decreasing itsradius. In this case, the sign (+/−) on the output of the signalcomparator shows whether to pump or release gas from the pressure tank.Such control can be used for keeping a bubble in resonance with thecentral frequency of a deepwater sound source. The same system can runcontinuously with a linear frequency modulated signal keeping the bubbleresonator in resonance with the instantaneous frequency of the radiatedsignal and keeping the phase of the radiated signal equal to the phaseof the control signal. The last approach can greatly expand frequencybandwidth of a frequency swept signal and make it useful for air-gunreplacement in the frequency band 5-100 Hz.

FIG. 5 shows a commercially available (or commercially off-the-shelf)air lift bag. Lift bags are available in very large diameters. Forexample, the lift bag depicted had more than a 1 meter radius which issuitable for a 20-30 Hz sound source. The bubble can be used as aflotation for a mooring design and can be filled with air continuouslyduring deployment. High pressure air hoses for 1500 meters are alsoavailable and can be used to pump air from a ship's deck.

FIG. 6 is an illustration comparing exemplary sizes of low frequencyorgan-pipe sound sources to a size of an exemplary bubble sound source.

FIG. 7 is a flowchart of a method of producing sound, according to anexemplary embodiment. At a step 810, an elastic bubble filled with a gasis provided into an underwater environment. The environment may be ashallow or deep underwater environment, depending on the use. At a step812, the gas within the bubble is perturbed in any of a variety of ways.At a step 814, the perturbing of the gas within the bubble is controlledto emit sound waves over a plurality of frequencies. The frequencies maybe discrete frequencies or a linear sweep of frequencies over a range offrequencies (e.g., a chirp). The perturbing may be computer-controlled.As described above, the resonant frequency of the bubble may further becontrolled with the computer or another computer by adjusting(increasing or decreasing) the volume or pressure of air within thebubble.

The exemplary embodiments have been described herein with reference to asymmetrical air pump with opened cylinders, a closed central partbetween the cylinders an driven by an electric motor. Other actuatorsare contemplated. FIG. 8A shows the first embodiment of a symmetricalforce-balanced mechanical actuator, which has a sealed cylinder 22 withtwo symmetrically moving pistons 4. This embodiment may be used in asystem sweeping in a large frequency bandwidth, and may be used in ashallow water application. FIG. 8B shows a second embodiment of asymmetrical force-balanced mechanical actuator, which has a Helmholtzresonator camera (similar to known low-frequency subwoofers) with anarrow throat 28. Unlike the embodiment of FIG. 8A, the embodiment ofFIG. 8B will have a resonant frequency in addition to the main bubbleresonance. The bubble resonant frequency and driver resonant frequencymay be closely grouped to form a doubly resonant projector. Thisprovision yields a transmitting bandwidth of ½ octave or greater. Thedriver for the bubble projector may supply greater blocked pressure andreduced volume displacement than the direct emitter when both areoperating at the same radiated power. The driver of FIG. 8B can be usedas a deep-water variant to expand the bandwidth of the system.

Preferably, the actuator will be balanced for internal forces to keepthe actuator from unexpected vibrations, in order to improve efficiency.Motor speed control may be done with any kind of modulation, such asphase modulation, frequency modulation, pulse code modulation (PCM),etc.

The bubble may be of a variety of different sizes. For example, thebubble may be at least 0.2 meters tall or in diameter or less than 4meters tall or in diameter.

The sound source may be configured to emit sound of at least about 5 Hz,or less than about 150 Hz, or preferably between 10 Hz and 150 Hz orbetween 10 Hz and 100 Hz.

The sound source may be configured to provide a sound pressure level ofgreater than or equal to about 216 dB re 1 uPa/Hz@1 m.

The sound source may be configured to have an efficiency of at leastabout 10%, or less than about 30%.

The sound source may be configured to keep the bubble in a predeterminedshape (e.g., sphere, fully inflated, etc.) under high water pressure ata depth of 1 kilometer or greater, or a depth of 1.5 kilometers orgreater. A frequency control loop can be used for keeping pressureinside the bubble under necessary values.

Processing circuit 14 may comprise analog and/or digital circuitcomponents, such as one or more microprocessors, microcontrollers,application-specific integrated circuits, interfaces, buses, A/Dconverters, etc. The circuit components may be configured or arranged toperform one or more of the functions or steps described herein, as wellas other functions related to or needed to perform the functions orsteps described herein. In one example, processing circuit 14 maycomprise a non-transitory computer-readable storage medium, such as amemory, encoded with computer instructions that, when executed by aprocessor, perform the functions or steps described herein.

In other embodiments, a tunable resonant sound source with rigid andelastomeric portions may be used. The tunable resonator may be a tunablepipe resonator configured to include rigid, movable sleeves at bothsides of the resonator. The tunable pipe and the movable sleeves may becylindrical in shape. The sleeves may be connected to a mechanicallinear actuator configured to move the sleeves to tune the controlfrequency of the resonator to the resonant frequency of the emittedsound waves and to keep the phase of the radiated signal equal to thephase of the control signal. The resonator may be configured to includeinside the cylindrical portion a gas-filled bubble comprising an elasticmaterial. An acoustical emitter, such as a volume velocity actuator, maybe configured to perturb the gas within the bubble. The acousticalemitter may be disposed within the gas-filled bubble.

In some embodiments, the tunable resonator may include a cylindricaltunable pipe filled with gas and configured to include an internalcoaxial pipe separating the internal volume of the resonator into twoparts. The volumes may be configured to have at least one orificebetween them. The orifice may be permeable to gas within the twovolumes. A moving sleeve, which may be attached to an electrical motoractuator to enable sliding along the internal pipe, may cover theorifice between the volumes. An elastic membrane may be disposed on twosides of the cylindrical tunable pipe. One of the internal volumes maybe separated from water by the elastic membrane. The other internalvolume may be rigidly closed. An acoustical emitter, such as a volumevelocity actuator, may be disposed between the two volumes. Theacoustical emitter may be configured with two pistons symmetricallydriven by an electrical motor. The tunable resonator may also include aprocessing circuit, including a microphone inside the gas bubble. Theprocessing circuit may be configured to provide a control signal to theacoustical emitter and control signal to the actuator to cause theactuator to move the sleeves. Moving the sleeves may tune the resonatorand keep it in resonance with the emitted sound waves over a pluralityof frequencies.

Referring to FIG. 9, a tunable pipe sound source or resonator 915 isshown. One or more of the elements of the embodiment of FIG. 9 may havesimilar structure and/or function as described with respect tocorresponding elements in FIG. 1. The tunable pipe sound source 915 mayinclude portions that are exposed to water. Sound source 915 may includerigid portions and elastomeric portions, both of which are exposed towater. Sound source 915 may include a rigid portion, such as a centraltube 96, and an elastomeric portion, such as gas-filled bubble 91. Inone embodiment, the tunable pipe sound source 915 may include a centraltube 96 with rigid, movable coaxial sleeves 917. Central tube 96 may bea fixed rigid member, and sleeves 917 may be rigid movable members. Inone embodiment, central tube 96 and coaxial sleeves 917 are cylindrical.In other embodiments, the central tube and coaxial sleeves may bedifferent shapes. Central tube 96 may have an axis 916 through thecenter of the tube, along the length of the tube. Movable sleeves 917may be disposed along the same axis 16 as central tube 96. In otherembodiments, central tube 96 may have one or more axes, laying indifferent directions. Sleeves 917 may be moved laterally by linearelectrical actuator 98, which may be controlled by microcontroller 911.In one embodiment, the sleeves may slide on wheels (not shown) along theinternal pipe 96. Sound source 915 may include bubble 1 in the middle ofcentral tube 96. Bubble 91 may be filled with a gas. The gas may be air,Nitrogen, or other gases.

Gas-filled bubble 91 may contain an acoustical driver 95 inside. Invarious embodiments, acoustical driver 95 may be a volume velocityactuator or a symmetrical pressure-balanced volume driver. Acousticaldriver 95 may be configured with a plurality of symmetrically movingpistons 94, which are connected to crank 93 via rods 918. Crank 93 maybe driven by electrical motor 92. In other embodiments, different typesof motor, e.g., linear electrical motor, linear actuator, linear movingmagnet actuator, variable reluctance motor, or linear voice coilactuator may be utilized. In other embodiments, pistons may be driven bymechanisms other than crankshaft mechanism. Two moving pistons 94 areprovided in this embodiment, though three, four, or more pistons orother moving members may be used in alternative embodiments. Electricalmotor 92 may be controlled by an electrical driver 910 and synchronizedwith a digitally synthesized signal waveform. At least a portion of thegas-filled bubble 91 may comprise an elastic material, which benon-transparent for gas under pressure. The tunable pipe sound sourcemay be equipped with a microphone 99 inside air-filled bubble 91.Microphone 99 may be connected through amplifier 913 with phasecomparator 914. The output of phase comparator 914 may be connected toan analog-digital converter (ADC) of microcontroller 911 with a preciseclock 912. Precise clock 912 may be, e.g., a temperature-compensatedcrystal oscillator (TCXO) or a Rubidium or Cesium atomic clock.

To change the resonant frequency of the resonator, the length ofresonant pipe may be increased or decreased by moving the two coaxialtubular sleeves 917 in opposite directions along the main pipe 96. Insome embodiments, both main pipe 96 and sleeves 917 may be rigid. Mainpipe 96 may be considered a first rigid portion, and sleeves 917 may beconsidered a second rigid portion. The movement of the second rigidportion relative to the first rigid portion may change the resonantfrequency of the bubble. When cylindrical sleeves 917 slide outwardlyand increase the length of resonator, the resonant frequency maydecrease, as inverse square-root dependence, because of an increasingmass and inertia of water inside the sleeves. When cylindrical sleeves917 slide inwardly and decrease the length of the resonator, theresonant frequency may increase, as inverse square-root dependence,because of a decreasing mass and inertia of water inside the sleeves.Extending the sleeves may be done as an alternative to changing thedimensions of bubble 91 to change resonant frequency. The movablesleeves 917 may be attached to one or more linear electric motoractuators 98, which control the position of the sleeves and ultimatelythe resonant frequency of the resonator. The gas-filled bubble 91 insidethe resonator contains microphone 99 connected to a phase comparator914. The phase comparator 914 may compare the phase from microphone 99(i.e., the phase of the emitted sound waves) with the phase of areference signal from microcontroller 911 (i.e., input signal drivingacoustical driver 95). The microcontroller 911 of the system, whichreceives the output of the phase comparator 914, may compensate for thephase difference by moving the sleeves 917. Moving the sleeves 917 maykeep the system in resonance with the instant frequency of the radiatedsignal.

The resonator may use a phase locked loop to track the phase offrequency swept signals. The sound source of FIG. 10 may transmitfrequency swept signals with an arbitrary law of frequency change in awide band. The frequency can change continuously with an arbitrary rate(frequency/time). The difference between frequencies of the signals maybe arbitrary. The difference between frequencies during the sweep may befixed or may vary. When frequency sweeping, the frequency response ofpropagation media (e.g., water) may be measured. It may be beneficial tosweep with a smaller rate of frequency change when frequency response ischanging rapidly with frequency. The resonant frequency may be tunedfrom 100 Hz down to 5 Hz by extending and retracting the sleeves. Thesound source of FIG. 10 may have a low resonant frequency (5 Hz andlower) and a high Q (up to 50), when working at depths smaller than 100m.

The sound source of FIG. 9 may be a tunable high-Q resonator. Thesymmetrically moving pistons 94 driven by electrical motor 92 through acrankshaft mechanism may serve as one example of a powerful volumeacoustic driver 95. The acoustic driver 95 may be limited in power onlyby the power of the electric motor 92 and can reach a maximum value of ahundred kW. The source of FIG. 9 may have the advantage of not changingthe dimensions of the gas-filled bubble, and thus the buoyancy of theresonator, when changing resonant frequency. Such transducer may bestably towed by, e.g., a ship during frequency sweeping. Constrainingthe gas-filled bubble 1 inside resonator tube may make the design morerigid, which also makes the resonator suitable for towing. The frequencytuning mechanism may be quick, with an expected maximum rate offrequency change of 30-50 Hz/sec.

The sound source of FIG. 9 may have 1-2 dB directivity gain in thedownward direction and may have practically no wall vibrations. Allforces in the sound source may balance each other and potentially willhave a much higher efficiency than a non-symmetrical variant. Thecylindrical form of the seismic transducer and small weight of the soundsource may increase overall towing stability. The bubble source is lesslikely to be damaged by cavitation. A gas-filled bubble oscillating inwater is less likely to blow apart or split the water, and make vacuumcavities that cause cavitation damage. To sweep over a large frequencyband, the high-Q resonator may be tuned by moving coaxial sleevessymmetrically and enlarging the length of the resonator.

The resonant frequency of the gas-filled bubble sound source may becontrolled. The resonant frequency and phase of the radiated signal maybe kept equal to the frequency and the phase of the control signal bycomparing the phase between an electrical signal from the internalmicrophone 99 (FIG. 9) and the control signal. The internal soundpressure of the sound source may be proportional to the signal from themicrophone 99. FIG. 10A contains a plot of the frequency of the signalfrom the microphone compared to the sound pressure inside the bubble.FIG. 10A depicts the sound pressure for a sound source with a resonantfrequency around 8.25 Hz. Curve 1010 may be the sound pressure forcontrol signals varying from around 1 Hz to around 15 Hz. Curve 1010 mayhave a peak 1020. Peak 1020 may occur when the frequency and phase ofthe control signal are equal to the resonant frequency and phase of thesound source (i.e., around 8.25 Hz). The sound pressure of the soundsource may be greatest, if only the control frequency is varied, whenthe control signal equals the resonant frequency of the sound source.

The phase of the microphone signal can be compared by the phasecomparator 914 with the phase of the sound source input signal frommicrocontroller 911 (FIG. 9). FIG. 10B contains a plot of the phasedifference between the radiated signal and the control signal (i.e., theinput signal to acoustical driver 95 of FIG. 9) as a function of thefrequency of the control signal. The plot of FIG. 10B may correspond toa sound source with a resonant frequency around 8.25 Hz. Curve 1030 maybe positive (as it is on the left side of FIG. 10B) if the phase of theradiated signal of the sound source (i.e., the phase of the microphonesignal) is greater than the phase of the control signal. If themicrophone phase is larger than the control signal phase, then theresonant frequency of the resonator is higher than the frequency of thesignal. To decrease the resonant frequency, sleeves 917 (FIG. 9) may bemoved to extend the length of the transducer. Curve 1030 may be negative(as it is on the right side of FIG. 10B) if the phase of the radiatedsignal of the sound source (i.e., the phase of the microphone signal) isless than the phase of the control signal. If the phase of themicrophone signal is lower than the phase of the input signal, then theresonant frequency of the transducer should be increased by moving thesleeves to the center and decreasing the transducer length. Curve 1030may reach a resonance point 1040 when the phase of the sound source isequal to the phase of the control signal. Resonance point 1040 may occurwhen the frequency of the control signal equals the resonant frequencyof the sound source (i.e., the phase difference is zero). In FIG. 10B,the resonant frequency of the sound source may be around 8.25 Hz. Thesign of the phase difference is different before and after resonance.The sign of the output of the signal comparator may show how to move thesleeves. If the output is positive, the sleeves may be moved outwardly(i.e., extend sleeves) to reach resonance. If the output is negative,the sleeves may be moved inwardly (i.e., move sleeves towards centralpart of tube) to reach resonance.

Phase locked loop (PLL) may allow for the tunable source to be kept inresonance with instant frequency of the control signal and for the phaseof the radiated signal to be kept equal to the phase of the controlsignal. This control may be used for keeping the resonator in resonancewith the frequency of emitted waves from the sound source. The samesystem can run continuously with the linear frequency modulated signal(or any other given frequency modulation law), keeping the bubbleresonator in resonance with the instant frequency of the radiated signaland keeping the phase of the radiated signal equal to the phase of thecontrol signal. The proposed approach can substantially expand thefrequency bandwidth of the frequency swept signal and make it useful forair-gun replacement in the frequency band of at least about 5 Hz toabout 100 Hz or less, or other frequencies.

FIG. 11 is an electrical circuit that may simulate the tunable resonatorof FIG. 9. Mathematical models of the acoustic properties of a bubblesound source allow for prediction and determination of the mainparameters of the bubble sound source. The standard electrical circuitmodel of acoustical structures suggests an equivalence of sound pressureP to the electrical voltage V and volume velocity V, to the electricalcurrent I. In that case, compliance of the medium (e.g., water) willcorrespond to an electrical capacitor with capacitance C, and inertia ofwater mass corresponds to the electrical inductor with the inductance L.Equations may be derived to precisely show the rigorous logic of thephysical acoustics of an underwater gas filled bubble.

Values for the radiation resistor R_(w) and attached water inductanceL_(w) are:

$\begin{matrix}{{R_{w} = \frac{\rho \; c}{A_{b}}};} & (1) \\{{L_{w} = {\frac{\rho \; a}{A_{b}} = {R_{w}\frac{a}{c}}}},} & (2)\end{matrix}$

where A_(b)=4πa² is the bubble surface area, a is the bubble radius, ρis the water density, and c is the sound velocity. Compliance (capacity)of the bubble has the form for gas compressibility:

$\begin{matrix}{{C_{b} = \frac{V_{b}}{P_{b}\gamma}},} & (3)\end{matrix}$

where γ=1.4, the ratio of the specific heat at constant pressure to thespecific heat at constant volume for the gas within the bubble; P_(b) isthe pressure of the gas inside the bubble; and

$V_{b} = {\frac{4}{3}\pi \; a^{3}}$

is the bubble volume. The tunable variable inductance of water inertiainside the sleeve L_(t) is:

$\begin{matrix}{L_{t} = \frac{\rho \; l}{A_{t}}} & (4)\end{matrix}$

where l is the length of the throat of the resonator (length of extendedsleeve), and A_(t) is the area of the sleeve.

The frequency transfer function for the tunable resonator with thebubble transducer has the form

$\begin{matrix}{{{{V_{v}(t)} = \frac{{\omega}\; L_{t}I}{\left\lbrack {1 + {{\omega}\left( \frac{L_{w}}{R_{w}} \right)} + {({\omega})^{2}{C_{b}\left( {L_{t} + L_{w}} \right)}} + {({\omega})^{3}\frac{C_{b}L_{t}L_{w}}{R_{w}}}} \right\rbrack}},{where}}\text{}{L_{t} = \frac{\rho \; l}{A_{t}}}} & (5)\end{matrix}$

is the tunable variable inductance of water inertia inside the sleeve; lis the length of the throat of the resonator (length of extendedsleeve); A_(t) is the area of the sleeve;

$R_{w} = \frac{\rho \; c}{A_{a}}$

is the radiation resistor, where A_(a) is the area of aperture, ρ is thewater density, and c is the sound velocity;

$L_{w} = {R_{w}\frac{a}{c}}$

is the water inertia, where a is the radius of the resonator; and

$C_{b} = \frac{V_{b}}{P_{b}\gamma}$

is the compliance of the gas within bubble volume V_(b), P_(b) is thepressure of the bubble gas, and γ=1.4, the ratio of the specific heat atconstant pressure to the specific heat at constant volume for gas withinthe bubble.

The derived equations give the volume displacement of the acousticdriver and displacement of the stroke of the driving mechanism. From thebubble pressure, the force can be calculated as well. The model givesthe parameters of the acoustic driver and allows for the prediction ofthe sound pressure for the design. The results of the simulated soundpressure of the tunable projector are shown in the FIG. 12. FIG. 12shows the variety of different resonance curves for sound pressure level(SPL) corresponding to different displacements of sleeves changing inthe range from 0.01 m to 0.5 m. Resonance curve 1212 corresponds to acontrol signal frequency around 17 Hz. Curve 1212 illustrates thevariation in sound pressure of sound source 915 (FIG. 9) depending onthe displacement of sleeves 917. When sound source 915 reaches resonancewith the control signal around 17 Hz, curve 1212 experiences peak 1210.Peak 1210 corresponds to a position of sleeves 917 that achievesresonant frequency. Peak 1210 may correspond to a maximum sound pressureof sound source 915, when only displacement of sleeves 917 is varied.The parameters of the simulation are:

Sea depth: 30 m;Diameter of internal resonator: 0.5 m;Diameter of aperture: 1.25 m;Length of air filled cylinder: 2.0 m;Length of sleeves: 0.01 m-0.5 m.

Referring to FIG. 13A, a gas-filled tubular resonator and tunable pipesound source 1315 is shown, according to another embodiment. One or moreof the elements of the embodiment of FIG. 13A may have similar structureand/or function as described with respect to corresponding elements inFIG. 1. Sound source 1315 may be considered a tubular resonator. Soundsource 1315 may include exterior portions that are exposed to water. Theexterior of sound source 1315 may include rigid portions and elastomericportions, both of which may be exposed to water. The exterior of soundsource 1315 may include rigid portion, such as an outer wall 1320, andan elastomeric portion, such as rubber boots or membranes 131 on bothsides. In other embodiments, different portions of the exterior may berigid and elastomeric. Sound source 1315 may be filled with a gas. Theinterior of sound source 1315 includes tunable central pipe 136. Centralpipe 136 may be variously referred to as main tube/pipe, centraltube/pipe, internal tube/pipe, or tubular wall. In one embodiment,central pipe 136 may be tubular or cylindrical. Other shapes may be usedin other embodiments. Resonator 1315 may include a tunable mechanisminside rubber boots or membranes 131. Rubber boots or membranes 131 maycomprise an elastic material, which be non-transparent for gas underpressure. Resonator 1315 may be configured to include, inside, a coaxialcentral pipe 136 separating the internal volume into two parts, firstvolume 1316 and second volume 1317. First volume 1316 and second volume1317 may be filled with gas. The gas may be air, Nitrogen, or othergases. Central tube 136 may be configured to have at least one orificeor opening 1318 between it and moving sleeves 137. Central tube 136 maybe considered a rigid tubular wall. Orifices 1318 may allow for the flowof gas between first volume 1316 and second volume 1317. Central tube136 may have an axis 1319 through the center of the tube, along thelength of the tube. Movable sleeves 137 may be disposed along the sameaxis 1319 as central tube 136. In one embodiment, sound source 1315 mayinclude two movable sleeves disposed symmetrically along central tube136. In other embodiments, central tube 135 may have one or more axes,laying in different directions. Moving sleeves 137 may be attached tothe electrical motor actuator 138, which may enable the sleeves to movelaterally along the internal pipe 136. Sleeves 137 may be movedsymmetrically. In one embodiment, sleeves 137 may slide on wheels alongthe internal pipe 136. In one embodiment, sleeves 137 may becylindrical. Other shapes may be used in other embodiments. Movablesleeves 137 may be considered rigid tubular members. In someembodiments, both main pipe 136 and sleeves 137 may be rigid. Main pip136 may be considered a first rigid portion, sleeves 137 may beconsidered a second rigid portion. The movement of sleeves 137 relativeto orifices 1318 of central tube 136 may change the resonant frequencyof the bubble. In one embodiment, both first volume 1316 and secondvolume 1317 may be filled with a gas, and elastic membrane 131 and outerwall 1320 may be in contact with water.

Resonator 15 may include an acoustical driver 135 in a wall between thetwo internal volumes. In some embodiments, acoustical driver 135 may bea symmetrical pressure-balanced volume driver or volume velocityactuator. Acoustical driver 135 may be configured with a plurality ofsymmetrically moving pistons 134, which are connected to crank 133 viarods 1321. Crank 133 may be driven by electrical motor 132. In otherembodiments, different types of motor, e.g., linear electrical motor,linear actuator, linear moving magnet actuator, variable reluctancemotor, or linear voice coil actuator may be utilized. In otherembodiments, pistons may be driven by mechanisms other than crankshaftmechanism. Two moving pistons are provided in this embodiment, thoughthree, four, or more pistons or other moving members may be used inalternative embodiments. Electrical motor 132 may be controlled by anelectrical driver 1310 and synchronized with a digitally synthesizedsignal waveform. The resonator 1315 may be equipped with the microphone139 inside first volume 1316. In various embodiments, the microphone maybe placed in different locations inside resonator 1315. Microphone 139may be connected through amplifier 1313 with phase comparator 1314. Theoutput of phase comparator 1314 may be connected to an analog-digitalconverter (ADC) of microcontroller 1311 with a precise clock 1312.Precise clock 1312 may be, e.g., a temperature-compensated crystaloscillator (TCXO) or a Rubidium or Cesium atomic clock.

The mechanism for tuning the resonant frequency of resonator 1315 maydepend on the inertia of the gas in the path of gas flow between firstvolume 1316 and second volume 1317. A gas flow path between the twovolumes may be created by orifices 1318. The gas flow path includes thegap between sleeves 137 and internal cylinder 136, and the area oforifices 1318. The volume and pressure of gas in the two volumes maychange during each half period of acoustical driver 135. During thefirst half of the frequency period, pistons 134 (of acoustical driver135) are moving from second volume 1317 to first volume 1316. As aresult, the pressure inside first volume 1316 may rise, and gas may flowfrom first volume 1316 to second volume 1317. During the second halfperiod, pistons 134 move from first volume 1316 to second volume 1317.As a result, the pressure inside second volume 1317 may rise, and gasmay flow back from second volume 1317 to first volume 1316. The rate(speed) of the gas flow determines the rate of the resulting pressurechange in both volumes and, consequently, the resonant frequency of thesystem. Increasing the rate of gas flow will increase the resonantfrequency of resonator 1315; decreasing the rate will decrease theresonant frequency. The rate of gas flow may depend on the inertia ofthe gas in the path between the two volumes. The inertia of the gas maybe adjusted by changing the area and length of the gas flow path. Lengthand area are dimensions of the gas flow path and depend on the positionof sleeves 137. The inertia of the gas is directly proportional to thelength of the path and inversely proportional to the area of the path.Moving sleeves 137 outwardly (relative to acoustical driver 135)decreases the length and increases the area of the gas flow path. Thus,moving the sleeves outwardly may decrease the inertia of the gas andincrease the resonant frequency of resonator 1315. Moving sleeves 137inwardly (relative to acoustical driver 135) increases the length anddecreases the area of the gas flow path. Thus, moving the sleevesinwardly increases the inertia of the gas and decreases the resonantfrequency. This tuning mechanism may allow for control of the resonantfrequency from inside the bubble without the exposure of moving parts tothe outside (e.g., water).

Resonator 1315 contains microphone 139 connected to a phase comparator1314. The phase comparator 1314 compares the phase from the microphone139 (i.e., the phase of the emitted sound waves) with the phase of areference signal from microcontroller 1311. The microcontroller 1311 ofthe system, which receives the output of the phase comparator 1314, maycompensate for the phase difference by moving the sleeves 137. Movingthe sleeves 137 may keep the system in resonance with the instantfrequency of the transmitted signal. The resonator can use a phaselocked loop to track phase of frequency swept signals. The operation ofmicrophone 139 and phase comparator 1314 to tune the resonant frequencyis similar to the operation described in the discussion of FIGS.10A-10B, above. The sound source of FIG. 13A may transmit frequencyswept signals with the arbitrary law of frequency change in a very wideband. The resonant frequency may be tuned from 100 Hz down to 5 Hz byextending and retracting the sleeves. The sound source of FIG. 13A mayallow for the frequency band covered by a resonator to be enlarged bytuning a narrow-band resonator over a large range of frequencies.

The sound source of FIG. 13A may have no moving parts outside of thehousing or in contact with the surrounding water. This may allow for theuse of lighter sleeves and decreased inertia (faster reaction) of thetuning mechanics. This may result in the ability for quicker frequencychange. The tuning actuator will work inside the gas chamber and may bemore reliable. The sound source operates with a larger radiationaperture and can be more powerful than other designs. The resonantfrequency may also be tuned without changing the dimensions of thebubble and its buoyancy, which may allow for stable towing by, e.g., aship during frequency sweeping.

FIGS. 13B-13F are alternate embodiments of sound source 1315 of FIG.13A. One or more of the elements of the embodiments in FIGS. 13B-13F mayhave similar structure and/or function as described with respect tocorresponding elements in FIG. 13A. FIGS. 13B-13F may not show all ofthe elements of FIG. 13A, and FIG. 13A may not show all of the elementsof FIGS. 13B-13F.

Referring to FIG. 13B, a cross-sectional view of sound source 1315 isshown, according to an exemplary embodiment. Sound source 1315 includesrubber boots or membranes 131. Rubber boots or membranes 131 may be anelastomeric portion of sound source 1315. Sound source 1315 alsoincludes outer wall 1320. Outer wall 1320 may be a rigid portion ofsound source 1315. Sound source 1315 is also shown to include centraltube 136. Central tube 136 may be considered a rigid tubular wall.Central tube 136 may divide the volume inside sound source 1315 intofirst volume 1316 and second volume 1317. Central tube 136 includes aplurality of orifices 1318. Orifices 1318 may allow for the flow of gasbetween first volume 1316 and second volume 1317. Orifices 1318 may becovered and uncovered depending on the position of sleeves 137. Sleeves137 may be a moveable. Sleeves 137 may be a rigid tubular member ofsound source 1315. Sleeves 137 may be moved by actuator 138. Acousticaldriver 135 is shown to be disposed within sound source 1315.

Referring to FIG. 13C, a cross-sectional view of sound source 1315 isshown from an elevated angle, according to an exemplary embodiment.Sound source 1315 includes rubber boots or membranes 131. Rubber bootsor membranes 131 may be an elastomeric portion of sound source 1315.Sound source 1315 also includes outer wall 1320. Outer wall 1320 may bea rigid portion of sound source 1315. Sound source 1315 is also shown toinclude central tube 136. Central tube 136 may divide the volume insidesound source 1315 into first volume 1316 and second volume 1317. Centraltube 136 includes a plurality of orifices 1318. Orifices 1318 may allowfor the flow of gas between first volume 1316 and second volume 1317.Orifices 1318 may be covered and uncovered depending on the position ofsleeves 137. Sleeves 137 may be a rigid, moveable member of sound source1315. Pistons 134, which form part of the acoustical driver, is shown tobe disposed within sound source 1315. In the embodiment of FIG. 13C,sound source 1315, central tube 136, and sleeves 137 are cylindrical inshape. In other embodiments, sound source 1315, central tube 136, andsleeves 137 may be different shapes.

Referring to FIG. 13D, the exterior of sound source 1315 is shown froman elevated angle, according to an exemplary embodiment. Sound source1315 includes rubber boots or membranes 131. Rubber boots or membranes131 may be an elastomeric portion of sound source 1315. Sound source1315 also includes outer wall 1320. Outer wall 1320 may be a rigidportion of sound source 1315. Sound source 1315 is shown to becylindrically-shaped in the embodiment of FIG. 13D. In otherembodiments, sound source 1315 may take other shapes.

Referring to FIG. 13E, a cross-sectional view from directly above soundsource 1315 is shown, according to an exemplary embodiment. Sound source1315 includes rubber boots or membranes 131. Rubber boots or membranes131 may be an elastomeric portion of sound source 1315. Sound source1315 also includes outer wall 1320. Outer wall 1320 may be a rigidportion of sound source 1315. Sound source 1315 is also shown to includecentral tube 136. Central tube 136 may divide the volume inside soundsource 1315 into first volume 1316 and second volume 1317. Central tube136 includes a plurality of orifices 1318. Orifices 1318 may allow forthe flow of gas between first volume 1316 and second volume 1317.Orifices 1318 may be covered and uncovered depending on the position ofsleeves 137. Sleeves 137 may be a rigid, moveable member of sound source1315. Motor 132 and pistons 134, which form part of the acousticaldriver, are shown to be disposed within sound source 1315.

Referring to FIG. 13F, a cross-sectional view through the side and alongthe length of sound source 1315 is shown, according to an exemplaryembodiment. Sound source 1315 includes outer wall 1320. Outer wall 1320may be a rigid portion of sound source 1315. Sound source 1315 is alsoshown to include central tube 136 and sleeves 137. Sleeves 137 may bemoved by actuator 138. Motor 132 and pistons 134, which form part of theacoustical driver, is shown to be disposed within sound source 1315.

FIG. 14 is a simplified electrical circuit that may simulate the tunableresonator of FIG. 13A. The use of mathematical models of the acousticproperties of the bubble source to predict the main parameters of thebubble sound source was described in the description of FIG. 11 above.The frequency response V_(v)(t) for the tunable bubble transducer is:

$\begin{matrix}{{{V_{v}(t)} = {\frac{{j\omega}\; L_{t}}{\left( {1 + {\left( {\frac{l}{j\; \omega \; C_{t}} + {{j\omega}\; L_{t}}} \right)\left( {{{\omega}\; C_{b}} + \frac{1}{j\; \omega \; L_{w}} + \frac{1}{R_{w}}} \right)}} \right)}I}},} & (6)\end{matrix}$

is the radiation resistor, where A_(a) is the area of aperture, p is thewater density, and c is the sound velocity;

$L_{w} = {R_{w}\frac{a}{c}}$

is the water inertia, where a is the radius of resonator;

$C_{t} = \frac{V_{2}}{P_{b}\gamma}$

is the compliance of gas within the volume V₂ behind the piston driver,where P_(b) is the gas density;

$L_{t} = \frac{\rho_{g}l}{A_{t}}$

is the tunable variable inductance of gas between resonators, where l isthe length of the path between the gas filled resonators, A_(t) is thearea of such path, and ρ_(g) is the gas density;

$C_{b} = \frac{V_{b}}{P_{b}\gamma}$

is the compliance of gas of the main bubble volume V_(b); V_(b) is themain bubble volume; V₂ is the volume of second resonator behind thepiston driver; and γ=1.4, the ratio of the specific heat at constantpressure to the specific heat at constant volume for gas within thebubble.

The results of the simulated sound pressure of the tunable projector areshown in the FIG. 15. FIG. 15 shows the variety of different resonancecurves for sound pressure level (SPL) corresponding to differentdisplacements of sleeves changing in the range from 0.01 m to 0.5 m. Theresonance curves of FIG. 15 are similar to the resonance curvesdiscussed in the description of FIG. 12. The parameters of the modelare:

Sea depth: 30 m;Diameter of internal resonator: 1.5 m;Diameter of aperture: 2.0 m;Length of air filled cylinder: 3.0 m;Length of sleeves: 0.01 m-0.5 m.

Various embodiments discussed herein may have application in the 5Hz-100 Hz frequency range for underwater ocean acoustic tomography,long-range acoustic navigation and communications and deep-bottompenetration seismic profiling in the offshore oil and gas industry.Variously embodiments may also be used in the 10 Hz-100 Hz frequencyrange for Artic/Antartic under-ice acoustic far-range navigation andcommunications, underwater global positioning systems (RAFOS), andlong-range ocean acoustic tomography and thermometry. Variousembodiments may provide a high efficiency broadband source in a lowfrequency band 5 Hz-400 Hz with a reasonably low cost. Variousembodiments discussed herein may be used above water. Variableembodiments discussed herein may not be tunable.

1. A sound source, comprising: a bubble configured to be filled with agas; a volume velocity actuator configured to perturb the gas within thebubble by changing the volume of gas within the bubble; a processingcircuit configured to provide a control signal to the volume velocityactuator to cause the volume velocity actuator to perturb the gas withinthe bubble at a frequency defined by the control signal; and a resonantfrequency control mechanism configured to keep a resonant frequency anda phase of a radiated signal of the bubble approximately equal to afrequency and a phase of the control signal.
 2. The sound source ofclaim 1, wherein the resonant frequency control mechanism comprises: arigid fixed member; at least one rigid movable member configured to moverelative to the fixed rigid member, the at least one rigid movablemember disposed substantially along the same axis as the fixed rigidmember, wherein the rigid movable member is coupled to an actuator; anda processing circuit configured to provide a control signal to theactuator to move the at least one rigid movable member.
 3. The soundsource of claim 2, wherein the at least one rigid movable member isconfigured to lengthen or shorten the sound source, the length of thesound source determining the resonant frequency of the sound source. 4.The sound source of claim 2, further comprising a sensor configured tosense a radiated signal of the bubble and to transmit the radiatedsignal to the processing circuit, wherein the processing circuit isconfigured move the at least one rigid movable member to keep theresonant frequency and the phase of the radiated signal of the bubbleapproximately equal to the frequency and the phase of the controlsignal.
 5. The sound source of claim 2, wherein the processing circuitis configured to detect a phase difference between the radiated signalof the bubble and the control signal, and to move the at least one rigidmovable member based on the phase difference.
 6. The sound source ofclaim 2, wherein the processing circuit is configured to control thesound source to perform a linear sweep of frequencies, wherein theprocessing circuit is further configured to move the at least one rigidmovable member to keep the resonant frequency and the phase of theradiated signal of the bubble approximately equal to the frequency andthe phase of the control signal across at least a portion of the linearsweep of frequencies.
 7. The sound source of claim 1, wherein the volumevelocity actuator is disposed within the bubble, the volume velocityactuator comprising an electro-mechanical motor comprising a pluralityof pistons symmetrically moving in opposite directions with a closedspace between the pistons, and wherein the volume velocity actuator isconfigured to change the volume and pressure inside the bubbleproportionally to the control signal.
 8. The sound source of claim 1,wherein the resonant frequency control mechanism comprises: at least onerigid movable member coupled to an actuator; and a fixed rigid membercontaining at least one opening, wherein the at least one rigid movablemember is movable relative to the at least one opening to define a gasflow path between a first volume and a second volume, wherein at leastone dimension of the gas flow path is determined by the position of theat least one rigid movable member; and a processing circuit configuredto provide a control signal to the actuator to move the at least onerigid movable member relative to the at least one opening to adjust theat least one dimension of the gas flow path.
 9. A sound source,comprising: a tubular resonator configured to be filled with a gas,wherein the exterior of the resonator comprises rigid and elastomericportions, and wherein an interior of the resonator comprises: a rigidtubular wall containing at least one orifice; a first volume and asecond volume, wherein the first volume and the second volume areseparated by the rigid tubular wall containing at least one orifice, theat least one orifice enabling a flow of the gas between the first volumeand the second volume; and at least one rigid tubular member configuredto move along the rigid tubular wall, wherein the position of the atleast one rigid tubular member regulates at least one dimension of thepath between the first volume and the second volume; a volume velocityactuator disposed within the resonator and configured to perturb the gaswithin the resonator; and a processing circuit configured to provide acontrol signal to the volume velocity actuator to cause the volumevelocity actuator to perturb the gas within the resonator at a frequencydefined by the control signal.
 10. The sound source of claim 9, whereinthe at least one dimension comprises an area of the path between thefirst volume and the second volume.
 11. The sound source of claim 9,wherein the at least one dimension comprises a length of the pathbetween the first volume and the second volume.
 12. The sound source ofclaim 9, wherein the position of the at least one rigid tubular memberdetermines the resonant frequency of a radiated signal of the resonator.13. The sound source of claim 9, wherein the rigid tubular member iscoupled to an actuator, further compromising a processing circuitconfigured to provide a control signal to the actuator to move the atleast one rigid tubular member relative to the at least one orifice. 14.The sound source of claim 9, further comprising a sensor configured tosense a radiated signal of the resonator and to transmit the radiatedsignal to the processing circuit, wherein the processing circuit isconfigured to move the at least one tubular member to keep a resonantfrequency and a phase of the radiated signal of the resonatorapproximately equal to a frequency and a phase of the control signal.15. The sound source of claim 9, wherein the processing circuit isconfigured to detect a phase difference between the radiated signal ofthe resonator and the control signal, and to move the at least one rigidtubular member based on the phase difference.
 16. The sound source ofclaim 9, wherein the processing circuit is configured to control thesound source to perform a linear sweep of frequencies, wherein theprocessing circuit is further configured to move the at least one rigidtubular member to keep the resonant frequency and the phase of theradiated signal of the resonator approximately equal to the frequencyand the phase of the control signal across at least a portion of thelinear sweep of frequencies.
 17. A method of generating underwater soundwaves, comprising: providing a tubular resonator configured to be filledwith a gas into an underwater environment, wherein end portions of thetubular resonator are covered by an elastic membrane, the tubularresonator comprising: at least two sections separated by a rigid tubularwall, the rigid tubular wall containing a plurality of openingsconnecting the two sections; at least two rigid tubular memberssymmetrically disposed along the rigid tubular wall, the at least tworigid tubular members configured to move relative to the plurality ofopenings; a volume velocity actuator disposed within the resonator andconfigured to perturb the gas within the resonator; and a processingcircuit configured to provide a control signal to the volume velocityactuator to cause the volume velocity actuator to perturb the gas withinresonator at a frequency defined by the control signal; and perturbingthe gas within the resonator; controlling the perturbing of the gaswithin the bubble to emit sound waves over a plurality of frequencies;and controlling a resonant frequency and a phase of the resonator toapproximately equal a frequency and a phase of the control signal. 18.The method of claim 17, wherein controlling a resonant frequency and aphase of a radiated signal of the resonator comprises symmetricallymoving the at least two rigid tubular members.
 19. The method of claim18, wherein symmetrically moving the two rigid tubular members changesthe area and length of the path of gas flow between the two sections ofthe resonator.
 20. The method of claim 17, wherein controlling theperturbing of the gas further comprises perturbing the gas to emit soundwaves over a linear sweep of frequencies and controlling a resonantfrequency and a phase of the resonator to approximately equal afrequency and a phase of the control signal across at least a portion ofthe linear sweep of frequencies.